1. Fields of the Invention
The present invention relates to a method for manufacturing a solar cell with a nanostructural film especially by means of an optical layer. The method of the present invention can be repeatedly done in a general environment without special and expensive equipment or devices. The nanostructural film can be directly affixed on the surface of the solar cell so that the solar cell generates the light concentration effect to increase routes through which the light inside the solar cell travel, effectively reduce the light reflectivity and enhance the photoelectric conversion efficiency thereof.
2. Descriptions of Related Art
The sun is the source of life, and human being cannot live without the sun. Although there are no immediately exhausted crises for the fossil fuels, e.g. oil or coal, on which the life around the world rely, the carbon dioxide emission from the excessive use of the fossil fuel has already caused the serious greenhouse effect to become the culprit in the earth's warming temperatures. Furthermore, since the price of crude oil continued to rise in recent years and nuclear power plant safety concern, looking for alternative energy sources has become imperative. Alternative energy sources, such as wind, hydro, geothermal, biodiesel, solar cells and so on, to be called as green energy, have attracted considerable attention over recent years, among which the solar cell is the most promising due to its high theoretical efficiency and mature technology.
The solar cell can transform the solar energy into electrical energy based on the photoelectric effect of materials. The photoelectric effect is the phenomenon that light shines into the material to increase conductive carriers. In terms of the semiconductor materials, as the energy of the light is larger than the energy gap of the semiconductors, the free elector-hole pairs are generated in the interior. However, these elector-hole pairs can be recombined soon or captured by the carriers in the semiconductors to become vanished. If an internal electric field is applied at this time, the carriers will be quickly led out before vanished. The internal electric field is generated in the joint interface between p-type and n-type semiconductors, and a so-called solar cell uses the internal electric field to extract effectively the current to induce the electricity.
However, currently the biggest problem of the solar cell is that its luminous efficiency cannot continuously be improved. When light shines on the surface of the solar cell, the large difference between the refractive index of air (refractive index, n=1) and of silicon substrate (refractive index, n=3.42) will generate a large amount of Fresnel reflection; in other word, part of the incident light is straightly reflected and the rest thereof is absorbed by the solar cell to generate electron-hole pairs. This factor not only causes an unideal photoelectric conversion efficiency of the solar cell but also increases power generation cost to obstruct the application and development of the solar cell in daily life.
In order to change the direction of the incident light from direct to oblique to reduce the surface reflection and increase the amount of light absorbed by solar cell, the single layer or multiple layers of the dielectric materials with the refractive index between that of the substrate and of air are coated on the surface of the traditional solar cell, for example, silicon nitride (Si3N4), silicon oxide (SiOx), and titanium oxide (TiOx). The method of destructive interference is used to achieve the effect of anti-reflection by the traditional solar cell and to enhance the photoelectric conversion efficiency. However, the traditional manufacturing processes for films must be done in a high vacuum environment e.g. semiconductor clean rooms. The thickness of films must be precisely controlled in the traditional manufacturing processes. Films with an adequate refractive index are lacking for use in the traditional manufacturing processes. All the aforesaid are the main disadvantages for the traditional processes for making solar cells. Applications of anti-reflection films on the solar cell are thus substantially restricted due to such traditional processes for making solar cells.